1. Field of the Invention
The present invention relates to the control of fuel burning devices in general and in particular relates to a primary control unit and ignitor using a microprocessor for fuel and ignition control and intelligent flame monitoring of a fuel burning device such as kerosene fired heaters.
2. Description of Related Art
While the control circuit of the present invention is generally applicable to fuel burning systems, for purposes of an exemplary illustration it will be described in connection with portable space heaters of conventional and well-known types frequently used, for example, by contractors for heating and drying purposes.
The nature of the fuel burning portable space heater does not constitute a limitation on the present invention. In general, such a space heater typically comprises an outer housing surrounding a combustion chamber. Air is introduced into the combustion chamber. A burner is located at one end of the combustion chamber. The burner normally has a fuel nozzle frequently incorporating eductor means providing jets of air to draw, mix, and atomize the fuel delivered by the nozzle. The nozzle, together with the eductors, discharge a combustible fuel-air mixture into the combustion chamber. An ignitor is provided to ignite the mixture and, after initial ignition, continuous burning occurs. Typically, during the continuous combustion, convection heat currents issue from the end of the heater opposite the burner and additional heat radiates from the surface of the heater housing.
Portable space heaters of the general type described are frequently provided with an ignition transformer and a motor. The motor normally runs a fan supplying air to the combustion chamber and the eductors and operates a fuel pump.
When the portable space heater is functioning properly, fuel burning will occur near the end of the combustion chamber at which the burner is located. In the event of improper air flow, however, the flame will move toward the opposite end of the combustion chamber, the combustion becoming unsteady and inadequate for proper heating. Under such a circumstance, it is desirable to shut down the heater. Inadequate air may result from a malfunction of the fan or a blocking of the passages for air into the combustion chamber.
Inadequate operation and possibly dangerous conditions may also be indicated by a lower than normal temperature of the burner flame, representing improper combustion conditions.
It is also desirable to shut down the portable space heater when there is a flame failure. This can occur by virtue of faulty ignition, a blockage of the fuel nozzle or exhaustion of the fuel supply.
In any case, the malfunction can cause insufficient or incomplete burning or a failure to burn issuing fuel, producing a dangerous existence of highly flammable liquid or noxious fumes. Prior art devices include a number of safety control circuits for fuel burning devices proposed to avoid the many and often disastrous results of improper burning or failure of flame in apparatus such as portable space heaters. For example, circuits have been proposed incorporating a thermal sensitive circuit breaker, a heating coil to energize the circuit breaker and a cadmium sulfide cell to monitor the flames of the fuel burning device. The heating coil and the cell are connected in parallel, the cell being shunted across the heating coil. The theory was that so long as the cadmium sulfide cell sensed a proper flame, insufficient current would pass through the heating coil to trip the thermal sensitive circuit breaker. In such a circuit, however, the cadmium sulfide cell would sometimes react to flame flicker causing the tripping of the circuit breaker when shutdown of the fuel burning device was not actually required.
In another embodiment, a thermal sensitive circuit breaker and heating coil were again used together with a cadmium sulfide cell. In this embodiment, a relay was provided having normally closed contact points in series with the heating coil. When the cadmium sulfide cell sensed a proper flame, the armature of the relay would react so that the normally closed contact points would open removing sufficient power from the heating coil to prevent tripping of the circuit breaker. Such a circuit, however, has proven expensive to manufacture, requiring a relay which is a relatively large component and is subject to mechanical failure of the relay.
In still another prior art embodiment, the safety control circuit is a solid-state circuit making it easy to apply in the most convenient manner. The circuit is reactive both to improper burning and loss of flame, irrespective of the cause. The circuit is designed in the preferred embodiment such that it is not reactive to ordinary flame flicker so that unnecessary and sometimes damaging shutdown of the fuel burning device will not occur. The circuit contains no relay or similar devices of prior art apparatus which have often been subject to mechanical failure. The circuit is simple and contains a small number of elements and is characterized by quick response in the event of improper burning or flame loss.
The device provides the solid-state flame sensing control circuit with first and second leads capable of providing a connection to a power source. The electrical power means of the fuel burning device is connected across the first and second leads in parallel, such electrical power means comprising a motor and an ignition transformer. In the preferred embodiment, a thermal sensitive circuit breaker has the contacts thereof connected in one of the leads and across the first and second leads is connected an additional lead incorporating, in series, a heating coil for actuating the circuit breaker, a first resistor, and a silicon controlled rectifier. Yet another lead is provided incorporating a second resistor and a cadmium sulfide cell.
This last-mentioned lead is connected at one end to that lead containing the heating coil, first resistor, and silicon controlled rectifier at a position between the heating coil and the first resistor. The other end of the lead containing the second resistor and the cadmium sulfide cell is connected to the second lead. Finally, the gate of the silicon controlled rectifier is connected by a lead to that lead containing the second resistor and the cadmium sulfide cell at a position between the last two mentioned elements. The silicon controlled rectifier gate lead contains a breakover device.
When the cadmium sulfide cell senses a proper flame, sufficient voltage for firing the breakover device does not exist and the gate of the silicon controlled rectifier is not energized. Therefore, the silicon controlled rectifier will, under such conditions, be rendered nonconductive and the heating coil will then carry an insufficient amount of current to trip the thermal sensitive circuit breaker. Upon the occurrence of a flame failure or improper burning, the resistance of the cadmium sulfide cell will responsibly increase to produce the firing voltage of the breakover device. Once this occurs, the gate of the silicon controlled rectifier will be energized and the silicon controlled rectifier will be rendered conductive, thereby permitting a flow of current through the heating coil sufficient to cause it to trip the thermal sensitive circuit breaker. As provided, the breakover device ensures that the circuit will not react to normal flame flicker. This invention is disclosed in U.S. Pat. No. 3,741,709.
It would be advantageous to have a primary control and ignitor using a microprocessor for fuel and ignition control and intelligent flame monitoring of a fuel burning device such as kerosene fired heaters. Furthermore, with program changes of the microprocessor, the control unit can be used with gas heaters. The objective is to provide a fast shutdown of the heater if normal combustion does not occur or is lost after being established. It is important to make the heater as safe as possible and to eliminate odors associated with incomplete combustion during an out-of-fuel condition. It is also important to have an out-of-fuel condition indicator and a visual indicator of the operational sequences of the control unit including fault codes and a low fuel code using an LED display.
It would also be advantageous to provide longer ignitor life and prevent common mode interference between the power supplies of the microprocessor and the ignitor. It would further be helpful to have a unit that would have an international control which could operate from 30 to 260 volt operation and 50 or 60 cycle.
Finally, it would be helpful to provide a control module that would result in less assembly time and production costs.